How frequently does the earth's magnetic field reverse and when can
we expect the next reversal?

David Nobes, a geophysicist at the University of Canterbury, responded.

Predicting the Earth's magnetic field is a bit like predicting the
weather for a day next year. We know what the general pattern will
be for the season of the year, but we cannot accurately predict the
weather for that day. The weather is complex, what we call
non-linear. The earth's magnetic field is also complex and
non-linear. Only recently have geophysicists been able to accurately
reproduce all of the main features of the Earth's magnetic field,
including reversals and excursions. Reversals occur when the Earth's
magnetic field turns off, then back on again in the opposite
direction. Excursions occur when the earth's magnetic field turns
off, then back on again, but in the same direction as before.

Reversals and excursions do not occur in a regular fashion and there are
several hundred thousand years between each event. We know of reversals
about 2.2 million years ago, 1.7 million years ago and 900,000 years ago.
The last reversal of the earth's field that is clearly recorded in the
geologic record occurred about 700,000 years ago.

There may have been short reversals or excursions since, but these are
not yet clearly defined. The field appears to turn off slowly at first, then
more quickly, and the actual reversal may take less than 100 years.
We don't know if there are any biological effects from reversals,
but navigation and some communications would be affected. Imagine what
would happen to your compass! It would point to the South after a
reversal.